Add missing ':' to Alternate code sections

This was done as:

    sgit -e 's/\(# Alternate code\)$/\1:/g' '*README.md'

The heading levels and other things to do with alternate code that are
being discussed have not been addressed here. That will come later. This
is, as noted, just adding the missing ':' where it was missing.

The command does NOT touch it when it is not prefixed with '# ' because
there are cases where normal lines end with 'Alternate code' (these will
be turned into links later on).

2012/konno/README.md

@@ -24,7 +24,7 @@ NOTE: N is an integer from 0 to 255.
 ./konno 30
 ```
 
-### Alternate code
+### Alternate code:
 
 An alternate version of this entry, [konno.alt.c](konno.alt.c), is provided.
 This alternate code is an unobfuscated version of the winning code.

2012/zeitak/README.md

@@ -28,7 +28,7 @@ try [incorrect.c](incorrect.c) and the program itself.
 NOTE: it prints an error and exits on the first nesting error so it will not
 detect multiple issues!
 
-### Alternate code
+### Alternate code:
 
 An alternate version of this entry, [zeitak.alt.c](zeitak.alt.c), is provided.
 The file [zeitak.alt.c](zeitak.alt.c) provides a version that has been slightly